For coating surfaces of different materials, the most common industrial processes make use of products applied in liquid phase containing solvents, which later on have to be removed through evaporation: this requires long manufacturing times. Moreover, the vapors of the solvent themselves can be harmful for operators' health, and they have to be removed or absorbed in suitable filters. The process requires a huge consumption of energy, in that the parts to be dried are usually heated to facilitate solvent evaporation, and sometimes even cooled to allow their manipulation when leaving the drying oven.
In industrial processes, using paints and glues that polymerize instead of materials requiring drying is convenient, in that curing occurs in shorter times, with a greater convenience and versatility of industrial processes. One of the known curing systems consists in irradiating the material to be cured with UV radiation having suitable wavelength and appropriate intensity.
Alternatively, instead of UV radiations, infrared (IR) radiations can be used. In the following, explicit reference will be made to UV radiation, without losing generality.
The ultraviolet radiations are commonly classified according to their emission spectrum:
UV-A: radiation with wavelength comprised between 400 and 315 nm and energy comprised between 3.1 and 3.94 eV. This is the band nearest to the blue band of visible light; it is moderately dangerous for sight.
UV-B: radiation with wavelength comprised between 315 and 280 nm and energy comprised between 3.94 and 4.43 eV. In natural sunlight UV-B radiations are scarce; they can provoke eye inflammation (photokeratitis), and are mainly responsible for skin aging, erythemas and risk of cancer. UV-B radiations catalyze many chemical reactions.
UV-C: radiation with wavelength comprised between 280 and 100 nm and energy comprised between 4.43 and 12.4 eV. They are normally not present in natural sunlight, and are very dangerous in case of human eye and skin exposure; they are used in germicidal lamps and in the light curing of chemical products.
UV-lamps are well known in the art; the known UV-lamps tend not to reproduce the whole spectrum of natural sunlight, but generate a spectrum with a strong component of UV light and a component of visible light. Known lamps are discharge lamps that consist of an ampoule or a glass or quartz tube containing a gas and at least two electrodes, between which gas ionization occurs. Through ionization, the gas emits photons according to the electro-magnetic emission spectrum of the gas itself. Supplemental electrodes for triggering may be present. In the known art, the most common lamps use mercury vapors, which is in liquid phase in the ampoule, and transitions to gas phase during the initial heating of the ampoule when ignited. The emitted electromagnetic radiation can have a wide spectrum going from far UV (UV-C) to infrared. Moreover, in the art ovens for the light curing of light-curable products making use of the above-described lamps are known.
The curing of chemical products catalyzed by light (light curing) has a peak of efficiency in correspondence of specific wavelengths. Chemical reactions are known which are catalyzed by red or green visible light (as, for example, the natural photosynthetic processes which occur in plants), while chemical reactions that are catalyzed by blue light are known (camphorquinone is often used as catalyst sensitive to wavelength between 440 and 480 nm). For industrial scopes, often substances sensitive to UV-B and UV-C wavelengths are used.
Exposing the substance, the reaction of which is to be catalyzed to only the radiations to which it is sensitive is clearly advantageous.
The main drawback of the known lamps is that, up to now, obtaining a UV emission only in the desired band was difficult, in that known lamps all have an important component of their emission ranging from infrared to visible light, and moreover to UV-B and UV-C. This entails a low efficiency, in that the emission of radiations of undesired wavelength has to be disposed of as heat, with negative effects on the device efficiency, the energy consumption and possibly the noise produced by fans. A second drawback is the short duration of the known lamps, which in best cases does not exceed 2000 hours of working time.
A third drawback is the slowness in switching up and re-switching up of the known lamps. As light emission is dependent on the temperature of the gas contained in the ampoule, known lamps reach their peak of emission several seconds after having been switched on. Moreover, in case of switching off, often some time has to elapse before one can re-switch on the lamp, so that keeping the lamp always on, even if it is not used is convenient, in order to prevent downtime.
Nowadays UV LED emitting radiations of a single desired wavelength are available.
Documents describing the use of UV LEDs to achieve these aims are known, as for instance:
US20110119949 of LUMINUS DEVICES INC describes a method using LEDs instead of traditional discharge lamps.
WO201131529 of Air Motion Systems describes a modular disposition of UV LEDs in order to obtain a high intensity of radiant power. LEDs are arranged inside a liquid-cooled structure, and the uniformity of light emission is obtained through the peculiar form of the reflector, which can be elliptic or parabolic, and of a lens in the form of a cylindrical bar made of an optically transparent material.
EP2601052 of IST METZ GMBH describes a method to manufacture modules comprising a plurality of LEDs, which can be arranged on a heat sink.
What is described above is the preferred embodiment. Nonetheless, in the field of known ovens, also infrared (IR) technology is used. In an alternative embodiment, the UV LEDs can be replaced with IR LEDs, emitting in the infrared band (600-1400 nm, preferably 800-1200 nm).